Search for Life

in the Solar System and Beyond

Author

J.Izquierdo, J.Bundeli

Published

May 13, 2023

Abstract
This visual story takes a general audience to appreciate the extraordinary fortune of finding life in the universe. For a more immersive experience, try reading in Morgan Freeman’s voice.

A world of extremes

To understand our Solar System can be quite challenging. For starters, our Solar System is huge. The Solar System is so big that kilometers are not very useful anymore. For the rest of our journey, we will be measuring distances with help of the speed of light (aprox 300.000 km/s)

If we try to compare planets, we have to observe multiple dimensions simulatenously. For example, temperatures, distances or densities that are unique on every planet. This would require different magnitudes that are incompatible with eachother (°C, Km, …).

These measurements are very different from planet to planet. If we tried to represent the planets with their real proportions, it would be impossible for your screen to show everything. For this, it is more useful to create “planet signatures”.

As you can see in Figure 2, signatures are unique for each planet and based on normalised data. This way we can compare dimensions like millions of kilograms and other extreme measurements, even if we have one huge planet compared to a tiny moon.

Figure 1: Planets Volume.
Earth, a dwarf planet

Earth is stricktly speaking, a dwarf-planet. Jupiter is more than twice as massive than the other planets of our solar system combined. (Laboratory 2023)

Mercury is still shrinking

Mercury is already the smallest planet in the solar system and it’s only getting smaller and denser. (Vicky Stein 2022)

Venus is too hot

Venus’ thick atmosphere traps heat creating a runaway greenhouse effect. (Vicky Stein 2022)

Figure 2: Planet signatures

Every planet is unique on its own way. Planets can be made of rock, metal, gas,… each with different densities. We believe that over millions of years, the Sun has attracted the heaviest particles on its way. As a result, we find small, heavy planets near the Sun, while the less dense and gigantic can be found on the edges of the solar system.

Planets may also have moons, rings or asteroids belts. In some planets rain is water, while in other it rains diamonts or molten metal! But the most special and rarest of all possible outcomes, is life!

Life is so unlikely that it is almost considered accidental.

Earth is a comfortable place for life

The only planet known to contain life is Earth. Life is the result of many improbably coincidences. The most remarkable is its distance from the Sun. This makes possible to find water just at the right temperature for life to thrive.

Our neighbours Mars and Venus, are a few “minutes” too close or far from the Sun. Mars’ water is permanently frozen, while Venus is hotter than a steamer pot.

Water world

There is water everywhere in the universe. However, it has to be liquid to sustain life (Vicky Stein 2022)

Backward Stinky Sunrise

Venus rotates backward on its axis, opposite to most planets in our solar system. Its sulfuric atmosphere smells like rotten eggs! (Lunar and Institute 2023)

Organic molecules are everywhere

Organics are complex carbon-based molecules found in living things, but which can be created by non-biological processes too (Vicky Stein 2022)

Supersonic winds!

Winds on Neptune can blow up to 1,500 miles per hour (2,400 km/h). Is all that energy coming from the sun, from the planet’s core, or gravitational contraction? Researchers are working to find out. (Vicky Stein 2022)

Figure 3: Mean planet temperatures by distance from the Sun (lightspeed)

There are many other factors to enable life, and each of them is required. A little bit too much radiation and you are fried. Decrease the gravity and the athmospere would disolve. Even the slightest change in the polarity of Earth’s core and a solar storm would put and end to life.

This makes us the luckiest tenants in the solar system. But, how about beyond the Solar System? The so-called “Exoplanets”, are they “Earth-like”?

Exploring the Vastness of our Universe: Exoplanets and their Discovery Methods

Our solar system is huge, as we have now seen. With today’s technologies we are able to get more information about our solar system and get a better understanding. Companies like NASA or ESA have been studying our universe for years and have made considerable progress in the last decades. But what exactly is the universe?

The universe and it’s exoplanets

The universe is unimaginably big and nobody can say exactly how big it really is. But we know that the visible universe is about 93 billion light years in diameter. The universe is a vast expanse, filled with billions of galaxies, each containing billions of stars. A large collection of stars and other celestial bodies form a galaxy. The galaxy we live in is called the Milky Way. Our exploration of the cosmos has revealed that many of these stars have their own planetary systems, known as exoplanets. Discovering and understanding these distant worlds is a fascinating endeavor that expands our knowledge of the universe and the possibilities of life beyond our own solar system. (“Das Universum” 2022)

Exoplanet Discovery Methods

Scientists have developed various methods to detect and study exoplanets. Three prominent methods with already many discoveries of exoplanets are Transit, Radial Velocity and Microlensing:

  • Transit Method: When an exoplanet passes in front of its host star from our point of view, it causes a slight dip in the star’s brightness. By monitoring these periodic brightness changes, scientists can infer the presence of an exoplanet and gather information about its size, orbit, and atmosphere. You can think of it like a solar eclipse, when the moon moves in front of the sun and blocks the light. (“Discovery Methods,” n.d.)

  • Radial Velocity Method: This method relies on the wobbeling of the start, which is caused by the orbiting planets. By measuring these tiny periodic shifts in the star’s velocity, scientists can deduce the presence and characteristics of the exoplanet. (“Discovery Methods,” n.d.)

  • Microlensing Method: In the vastness of space, massive objects like stars and planets have a profound impact on the fabric of space itself. This gravitational influence bends and redirects light, akin to a cosmic lens. These fascinating distortions unlock new perspectives for astronomers, offering a glimpse into the captivating mysteries of the universe. This temporary amplification provides valuable information about the exoplanet. (“Discovery Methods,” n.d.)

These are the 3 main methods by which most exoplanets have been discovered over time. Other methods, such as imaging or transit timing variations, will not be discussed here.

In the visualization below, you can now see the number of exoplanets that have been discovered by each method.

Potentially habitable exoplanets

7 out of our top10 most habitable exoplanets have been discovered by transit method

Figure 4: Cumulative exoplanets discovered, by method

(“Visualization Cumulative Exoplanets, by Method” 2023)

In search of habitable worlds: The top 10 most life-friendly exoplanets

Now that we have familiarized ourselves with the various methods used to discover exoplanets, let’s focus on the fascinating world of life-friendly exoplanets. The question of the existence of life outside our solar system holds a special fascination for us. Might there be other planets out there that have conditions similar to earth’s and where life would be possible?

The conditions that a planet must fulfill to be habitable are not precisely known. However, it is comparatively easier to define conditions under which a planet is very likely not habitable. Finding such conditions is important as it can help select, in an ensemble of potentially observable planets, which ones should be observed in greater detail for characterization studies. A planet should have a maximum radius that he can fulfill two constraints: surface conditions compatible with the existence of liquid water, and no ice layer at the bottom of a putative global ocean.

Results havw shown that for planets in the super-Earth mass range (1–12 times the mass of the earh), the maximum that a planet, with a composition similar to that of the Earth, can have varies between 1.7 and 2.2 times the radius of the earth. (Alibert 2013)

Below in the visualization are the top 10 exoplanets marked with their relative mass and radius compared to earth which are potentially habitable. (Huber 2015)

Gas giants

Gas giants are planets the size of Saturn or Jupiter, the largest planet in our solar system, or much, much larger. (“Planettypes” 2022)

Super Earth

Super Earths are typically terrestrial planets that may or may not have atmospheres. They are more massive than Earth, but lighter than Neptune. 9 out of our top 10 exoplanets are superearths. (“Planettypes” 2022)

Neptun-like

Neptunian planets are similar in size to Neptune or Uranus in our solar system. They likely have a mixture of interior compositions, but all will have hydrogen and helium-dominated outer atmospheres and rocky cores. (“Planettypes” 2022)

Terrestrial planets

Terrestrial planets are Earth sized and smaller, composed of rock, silicate, water or carbon. Further investigation will determine whether some of them possess atmospheres, oceans or other signs of habitability. (“Planettypes” 2022)

Figure 5: Mass and Radius of exoplanets compared to other exoplanets and planets of our solar system

The goldilocks

As we already have seen the definition of “habitable zone” is the distance from a star at which liquid water could exist on orbiting planets’ surfaces. Habitable zones are also known as Goldilocks’ zones, where conditions might be just right – neither too hot nor too cold – for life. (“The Habitable Zone” 2021)

Several factors play a role in ensuring that exoplanets have surface temperatures that are not too warm and not too cold. Two of them are on the one hand the temperature of the host and on the other hand the orbit radius (the radius to the star around which the planets revolve). How does this compare for our top 10 most habitable exoplanets to Earth and to other exoplanets? And how far away are the exoplanets?

Tau Ceti e: the nearest exoplanet with potential life

The journey to the nearest exoplanet Tau Ceti e would take about 22 years by spacecraft.

Astronomical unit AU

The astronomical unit is a unit of length, roughly the distance from Earth to the Sun and approximately equal to 150 million kilometres or 8.3 light-minutes. (“Astronomical Unit,” n.d.)

Figure 6: Distribution of exoplanets for distance to earth, orbital radius and host temperature

As we can see in this visualization, the distribution of exoplanets with the corresponding properties is very broad. Nevertheless, our top 10 exoplanets have similar properties in terms of orbital radius as our Earth. Also the temperatures of the respective stars (host), which the planets orbit, are similar to the temperature of the sun. On these planets life could be actually possible.

Let’s take care of our unique planet earth

Whether life exists beyond Earth is one of the most profound questions of all time. The answer will change us forever, whether it reveals a universe rich with life, one in which life is rare and fragile, or even a universe in which we can find no other life at all. (“The Search for Life” 2020)

Nevertheless, these planets are at an unthinkable distance and it will be very difficult to colonize another planet. Our earth is unique and can’t be replaced easily - we should take care of our planet earth!

Data, tools and inspiration:

References

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